1. Field of the Invention
This invention relates to a catalyst for the polymerization of olefins, a method of making the catalyst and a method of polymerization using the catalyst, more particularly, a solid catalyst comprising a mixture of an aluminoxane and a metallocene and the related method of making and using such a catalyst.
2. Description of the Prior Art
Catalysts for the polymerization of olefins containing a metallocene and an aluminoxane are known Examples of such catalysts are disclosed in several patents.
U.S. Pat. No. 3,242,099 disclosed a metallocene [bis(cyclopentadienyl)titanium dichloride] and an oligomeric aluminum compound [(Rxe2x80x94Alxe2x80x94O)] as a catalyst for olefin polymerization.
U.S. Pat. No. 4,542,199 disclosed a bis(cyclopentadienyl) transition metal alkylhalide with an aluminoxane as a catalyst for the polymerization and copolymerization of ethylene.
U.S. Pat. No. 4,769,510 disclosed a chiral stereorigid zirconocene with an aluminoxane as a catalyst for the polymerization of propylene.
U.S. Pat. No. 4,791,180 disclosed a reaction product between a metallocene and alumoxane to form a solid catalyst for the polymerization and copolymerization of ethylene.
U.S. Pat. No. 4,892,851 disclosed a combination of aluminoxane with a bridged metallocene in which one cyclopentadienyl ring is substituted in a substantially different manner from another cyclopentadienyl ring. This combination generated a highly syndiospecific catalyst which produced highly crystalline syndiotactic polyolefin.
U.S. Pat. Nos. 4,752,597 and 4,791,180 disclosed an olefin polymerization catalyst and a polymerization process for polymers of ethylene and copolymers of ethylene and alpha-olefins. The metallocenes disclosed are unbridged which would not be effective for propylene polymerization. The aluminum to transition metal molar ratio was about 12:1 to about 100:1. The metallocene was reacted with a pentane solution of MAO, i.e., a low molecular weight fraction of MAO, to produce a solid which was dissolved in toluene for polymerization of ethylene.
U.S. Pat. No. 5,122,491 disclosed a catalyst produced by a reaction product of solid MAO precipitated with n-decane from a toluene solution, i.e., a high molecular weight fraction of MAO, with a metallocene in toluene to which an organoaluminum compound is added. This catalyst is useful for polymerization of olefins, especially ethylene and 4-methyl-1-pentene.
European Patent Application 88900586.4 (Publication No. 0327649) disclosed a solid catalyst for olefin polymerization composed of a metallocene and an alumoxane and having an average particle diameter of 5 to 200 micrometers and a specific surface area of 20 to 1000 m2/g. An aluminoxane suspension in a solvent such as n-decane is contacted with a solution of a metallocene in a solvent such as toluene to form a solid catalyst which was used for polymerization of ethylene and ethylene copolymer.
Two of the most serious process problems standing in the way of commercialization of metallocene based technology for olefin polymerization are low polymer bulk densities and severe reactor fouling. The bulk densities for either syndiotactic or isotactic polypropylene obtained with metallocene systems range from 0.10 g/cc upwards. The low bulk density problem appears to be more severe for syndiotactic polypropylene (SPP) than for isotactic polypropylene (IPP). The bulk densities of IPP with conventional titanium (IV) based heterogeneous Ziegler-Natta catalysts are in the range 0.40-0.50.
Reactor fouling, defined as the tendency of the polymer to stick to the surface of the reactor components, is also very severe with metallocene based catalyst systems. The reactor fouling necessitates extraction of the reactor components with a solvent thus adding to the cost of polymer production and makes the process more elaborate. It is not clear how interdependent the high degree of reactor fouling and low polymer bulk densities are.
It would be advantageous to have a metallocene catalyst and a process for using the catalyst which reduces reactor fouling and gives a polymer having high bulk density. Further, it would be advantageous if such a process were effective for both isospecific and syndiospecific polymerization.
Accordingly, the present invention provides a catalyst for the polymerization of olefins which reduces reactor fouling and gives a polymer having high bulk density.
This and other objects are accomplished by a solid catalyst comprising:
a) a metallocene of the general formula:
Rxe2x80x3b(CpR4)(CpRxe2x80x24)MR*vxe2x88x922
where Rxe2x80x3 is a bridge imparting stereorigidity to the structure of the metallocene by connecting the two cyclopentadienyl rings, b is 0 or 1, Cp is a cyclopentadienyl ring, R and Rxe2x80x2 are substituents on the cyclopentadienyl rings and can be a hydride or a hydrocarbyl from 1-9 carbon atoms, each R and Rxe2x80x2 being the same or different, each (CpR4) and (CpRxe2x80x24) being the same or different, M is a transition metal of a Group IIIB, IVB, VB or VIB metal, R* is a hydride, a halogen or a hydrocarbyl from 1-20 carbon atoms, v is the valence of M; and
b) an aluminoxane compound of the following general formula:
Al2OR4(Al(R)xe2x80x94O)n
for a linear aluminoxane and
(Al(R)xe2x80x94O)n+2
for a cyclic aluminoxane, n being 4 to 20 and R being methyl or ethyl.
The present invention provides a metallocene catalyst and processes for making and using the catalyst, particularly in the production of crystalline polyolefins, especially polypropylene. Olefins, especially propylene, may be polymerized to form polyolefins in amorphous (atactic) or crystalline forms. Examples of crystalline forms are isotactic and syndiotactic.
Isotactic polypropylene contains principally repeating units with identical configurations and only a few erratic, brief inversions in the chain. Isotactic polypropylene may be structurally represented as 
The methyl groups attached to the tertiary carbon atoms of successive monomeric units on the same side of a hypothetical plane through the main chain of the polymer, e.g., the methyl groups are all above or below the plane.
Another way of describing the structure is through the use of NMR. Bovey""s NMR nomenclature for an isotactic pentad is . . . mmmm . . . with each xe2x80x9cmxe2x80x9d representing a xe2x80x9cmesoxe2x80x9d dyad or successive methyl groups on the same side in the plane. As known in the art, any deviation or inversion in the structure of the chain lowers the degree of isotacticity and crystallinity of the polymer.
A syndiotactic polymer contains principally units of exactly alternating stereoisomers and is represented by the structure: 
The methyl groups attached to the tertiary carbon atoms of successive monomeric units in the chain lie on alternate sides of the plane of the polymer.
In NMR nomenclature, this pentad is described as . . . rrrr . . . in which each xe2x80x9crxe2x80x9d represents a xe2x80x9cracemicxe2x80x9d dyad, i.e., successive methyl groups on alternate side of the plane. The percentage of r dyads in the chain determines the degree of syndiotacticity of the polymer. Syndiotactic polymers are crystalline and like the isotactic polymers are insoluble in xylene. This crystallinity distinguishes both syndiotactic and isotactic polymers from atactic polymer that is soluble in xylene.
A polymer chain showing no regular order of repeating unit configurations is an atactic polymer. In commercial applications, a certain percentage of atactic polymer is typically produced with the isotactic form.
As noted above, a metallocene compound can be used with an aluminoxane compound to form a catalyst for the polymerization of olefins. The metallocene should contain two cyclopentadiene rings and be of the general formula:
Rxe2x80x3b(CpR4)(CpRxe2x80x24)MR*vxe2x88x922
where Rxe2x80x3 is a bridge imparting stereorigidity to the structure to the metallocene by connecting the two cyclopentadienyl rings, b is 0 or 1, Cp is a cyclopentadienyl ring, R and Rxe2x80x2 are substituents on the cyclopentadienyl rings and can be a hydride or a hydrocarbyl from 1-9 carbon atoms, each R and Rxe2x80x2 being the same or different, each (CpR4) and (CpRxe2x80x24) being the same or different, M is a transition metal of a Group IIIB, IVB, VB or VIB metal, R is a hydride, a halogen or a hydrocarbyl from 1-20 carbon atoms, v is the valence of M. Preferably, b is 1 and Rxe2x80x3 is a hydrocarbyl radical, more preferably an alkenyl radical having one to four carbon atoms, a dialkyl germanium, a dialkyl silicon, an alkyl phosphine or amine radical, such as a dimethyl silyl radical, an ethylenyl radical or a isopropenyl radical and, most preferably, is an ethylene radical for isospecific metallocene and isopropenyl radical for syndiospecific metallocene. Preferably, (CpR4) is cyclopentadienyl or substituted cyclopentadienyl ring such that it is 3-t-butyl-cyclopentadienyl or indenyl and (CpRxe2x80x24) is a substituted cyclopentadienyl ring such that it is indenyl or fluorenyl. Preferably, M is a Group IVB metal, most preferably zirconium, which has a valence of 4. Preferably, R* is a halogen or alkyl, most preferably chlorine or methyl. Specific examples of metallocenes are isopropyl (fluorenyl)(cyclopentadienyl)zirconium Dichloride, isopropyl(2,7-di-t-butylfluorenyl)(cyclopentadienyl) zirconium dichloride and ethylene bis(indenyl)zirconium dichloride.
The metallocene is combined with an aluminoxane compound of the following general formula:
Al2OR4(Al(R)xe2x80x94O)n
for a linear aluminoxane and
(Al(R)xe2x80x94O)n+2
for a cyclic aluminoxane, n being 4 to 20 and R being methyl or ethyl. The preferred aluminoxane is methyl alumoxane (MAO).
The novel catalyst is a mixture of metallocene catalyst component and aluminoxane co-catalyst from which the solvent has been removed to form a solid. Use of this catalyst in olefin polymerization results in increased bulk density of the polyolefin. The new method of polymerization differs from the conventional method of aluminoxane/metallocene catalyzed polymerizations in the mode of introduction of the catalyst into the reactor. Conventionally, the catalyst is reacted with MAO supplied as a solution in a hydrocarbon solvent followed by injection of the resulting homogeneous solution into the reactor containing liquid olefin. In the novel method of making this catalyst, the solvent is removed from the mixture of metallocene and aluminoxane. The solid mixture is ground and delivered into the reactor in a mineral oil slurry. The new catalyst and new methods of making and using the catalyst are applicable to all stereospecific metallocene systems.
Introduction into the reactor of MAO/metallocene mixture as solvent-free solid slurried in mineral oil offers several advantages over the conventional method of using homogeneous solutions for catalyst delivery. Some of the advantages include
a) ease of catalyst delivery under plant conditions;
b) Aromatic solvent free polymer fluff;
c) consistently high bulk densities;
d) significant decrease in reactor fouling;
e) minimal post-polymerization reactor cleanup needed;
f) general applicability to all metallocene catalyzed olefin polymerizations;
g) polymer free of conventional inorganic support materials.